1. Field of the Invention
The present invention relates to an isolator and isolator manufacturing method.
2. Background Related Art
Various types of apparatuses, such as devices used for industrial or medical purposes, often require electrical insulation. For example, an electronic device controlled by high voltage is equipped with a signal isolator (isolator) so that when a signal received by the electronic device is transmitted to an external device or when the user directly manipulates the operation panel, the user is not subjected to electrical shock or other severe bodily harm. When the electrical potential difference between electronic devices or circuit blocks is large, the isolator has a function of transmitting a signal from the low voltage side to the high voltage side or from the high voltage side to the low voltage side, in an electrically isolated state where large currents are prevented from flowing between the low voltage side and the high voltage side (hereinafter, referred to as isolated transmission).
Meanwhile, even if the electrical potential difference between electronic devices or between circuit blocks is small, an isolator is disposed. For example, when an analog circuit and a digital circuit are connected at a common reference potential, the analog circuit is affected by digital noise. Therefore, by completely separating the reference potentials of the analog circuit and the digital circuit electrically by an isolator, digital noise can be prevented from being introduced at the analog circuit, thereby improving the performance of the signal processor.
Conventionally, photocouplers, which use light at a signal transmitter, are known as very versatile isolators. A photocoupler is configured by a photodiode and a phototransistor combined together as 1 package; where an input electrical signal is converted to light by the photodiode and the light/dark changes of this light are converted to voltage by the phototransistor to thereby perform electrically isolated signal transmission. Photocouplers offer the advantages of a simple package configuration and high electrical isolation performance. On the other hand, photocouplers cannot be fabricated (manufactured) on a semiconductor substrate by a general integrated circuit (IC) process. Consequently, photocouplers cannot be integrated with transmission circuits nor with reception circuits and are disposed as a separate component in a signal processor, making size reductions of signal processors difficult.
Further, the luminous efficiency of photodiodes temporally degrades consequent to operating conditions such as operating temperature and forward current. As a result, when the life expectancy of a photodiode-equipped device is of importance, the setting of conditions such as operating temperature and forward current must be carefully considered. In addition, the photocoupler has a slow response speed requiring periods on the order of microseconds for signal transmission. As a result, for example, an inverter drive circuit equipped with a photocoupler has to establish dead time on the order of microseconds for the device configured by the inverter and thus, increases in speed are not possible.
Next to photocouplers, coupling capacitors that use field variations consequent to capacitive coupling at the signal transmitter are known as isolators of high versatility. Coupling capacitors block direct current (DC) signals from the transmission circuit and transmit only alternating current (AC) signals to the reception circuit. Thus, coupling capacitors are effective when separating direct voltage settings of a circuit network between the transmission circuit and the reception circuit.
Further, coupling capacitors offer the advantages of high insulation resistance and low power consumption. Nonetheless, coupling capacitors have a problem of being easily affected by noise and external electric fields.
Transformers that use magnetic field variations based on inductive coupling with the signal transmitter are additionally known as isolators. Such transformers, in general, have a configuration in which a magnetic body of, for example, ferrite is disposed between 2 coils. However, despite having high insulation resistance and high resistance to noise, these transformers further have a high cost and high power consumption. Furthermore, since the coils in the transformers are large, a reduction in the size of the transformer is difficult. Like photocouplers, the coupling capacitors and the transformers are also equipped in signal processors as a separate component, making a reduction in the size of the signal processor difficult. Thus, with respect to a signal processor equipped with a photocoupler, a coupling capacitor, or a transformer, a problem arises in that reductions in the size of the signal processor become difficult.
A high voltage IC (HVIC) is known as a signal processor to address such problems. An HVIC can perform, via a level shift circuit, signal transmission between circuits having power sources of differing electric potentials and can be integrated with a transmission circuit and a reception circuit by general IC processing.
Consequently, reductions in cost, power consumption, and size can be facilitated. However, since the HVIC cannot perform isolated transmission of signals, tolerability of high voltages is difficult to facilitate, e.g., a voltage tolerance of a mere 1200V can be guaranteed. Further, since the HVIC has a low tolerance to noise and is easily damaged, the HVIC is not applicable to devices for which reliability is required.
Therefore, an isolator that can be integrated with a transmission circuit and a reception circuit onto the same semiconductor substrate by general IC processing, and that can perform isolated transmission of signals is demanded. Digital isolators are known as such an isolator. A digital isolator has a transformer that by a series of IC processes for fabricating a signal processor, is integrated with a transmission circuit and a reception circuit, and performs signal transmission using magnetic field variations based on the inductive coupling of 2 coils. Therefore, wide application of digital isolators as an isolator to be used in fields such as the industrial and medical fields is expected and the development digital isolators is actively progressing.
A digital isolator having a transformer in which solenoid coils that include magnetic bodies of, for example, ferrite, are arranged in parallel on a semiconductor substrate has been proposed as one such digital isolator. See, for example Wang, N., et al, “Thin Film Microtransformer Integrated on Silicon for Signal Isolation” (USA), IEEE Transactions on Magnetics, Vol. 43, No. 6, June 2007, pp. 2719-2721 (also referred to herein as “Non-Patent Literature 1”) and Xu, M., et al, “A Microfabricated Transformer for High-Frequency Power or Signal Conversion” (USA), IEEE Transactions on Magnetics, Vol. 34, No. 4, July 1998, pp. 1369-1371 (also referred to herein as “Non-Patent Literature 2”). Further, a digital isolator having a transformer in which 2 planar coils are layered via an insulating film on a semiconductor substrate, has been proposed as a digital isolator without magnetic bodies. See, for example, U.S. Pat. No. 7,683,654 (also referred to herein as “Patent Literature 1”), U.S. Pat. No. 6,927,662 (also referred to herein as “Patent Literature 2”) and U.S. Pat. No. 7,417,301 (also referred to herein as “Patent Literature 2”). See also, for example, Chen, B., “Isolated Half-Bridge Gate Driver with Integrated High-Side Supply” (Greece), IEEE Power Electronics Specialists Conference (PESC) 2008, June 2008, pp. 3615-3618 (also referred to herein as “Non-Patent Literature 3”), Kaeriyama, S., “A 2.5 kV Isolation 35 kV/us CMR 250 Mbps 0.13 mA/Mbps Digital Isolator in Standard CMOS with an on-Chip Small Transformer” (USA), IEEE Symposium on VLSI Circuits (VLSIC) 2010, June 2010, pp. 197-198 (also referred to herein as “Non-Patent Literature 4”), Munzer, M., “Coreless Transformer a New Technology for Half Bridge Driver IC's” (Germany), International Exhibition and Conference for Power Electronics, Intelligent Motion and Power Quality (PCIM)), May 2003 (also referred to herein as “Non-Patent Literature 5”), Analog Devices, Inc., “Datasheet of ADuM1234 (Analog Devices Application Note)”, 2007, pp. 1-10 (also referred to herein as “Non-Patent Literature 6”) and Infineon Technologies AG, “Datasheet of 2DE020I12-FI (Infineon Technologies Application Note)”, 2006 (also referred to herein as “Non-Patent Literature 7”).
As another digital isolator without magnetic bodies, a digital isolator having a transformer that has 2 coils formed of a metal film and embedded in 2 spiral-shaped, planar trenches formed in the same principal surface of the semiconductor substrate, has been proposed. See, for example, U.S. patent application Ser. No. 13/215,350 (also referred to herein as “Patent Literature 4”) and Rongxiang, W., “A Novel Silicon-Embedded Coreless Transformer for Isolated DC-DC Converter Application” (USA), IEEE 23rd International Symposium on Power Semiconductor Devices and ICs (ISPSD) 2011, May 2011, pp. 352-355 (also referred to herein as “Non-Patent Literature 8”).
However, the isolators of Non-Patent Literature 1 and 2 have cylindrical-shaped coils (solenoid coils) of spirally wound metal wire and because the thickness of the metal wire forming the coil cannot be increased, problems of increased DC resistance of the coil and decreased voltage gain arise. Furthermore, since overcurrent arises and the coil generates heat, a problem of large loss arises. In the isolators of the Patent Literature 1-3 and Non-Patent Literature 3-7, the thickness of the metal film forming the coils (planar coils) cannot be increased. Consequently, problems arise in that the DC resistance of the coil is high and the voltage gain is small.
In the isolator of Patent Literature 4 and Non-Patent Literature 8, the insulating film for electrically insulating the coil embedded in each of the 2 trenches is formed along a side wall of the trench and thus, the insulating film cannot be formed to be thick. Therefore, a problem arises in that high voltage tolerance becomes difficult.
Further, in the isolator of Patent Literature 4 and Non-Patent Literature 8, because the 2 coils oppose one another at the side wall of the trench, the area of the opposing portions is greater than that of other isolators. As a result, large parasitic capacitance arises and the isolator is easily affected by noise.
Thus, there are certain shortcomings in the related art.